It is widely recognized that bacteria cultured in vitro using conventional media and conditions express characteristics that are different from the characteristics expressed during growth in their natural habitats, which includes in vivo growth of normal microflora or pathogens in an animal host. Therefore, such in vitro grown pathogenic bacteria might not be good for use as vaccine components. However, if it were possible to define conditions that trigger or enhance expression of virulence factors, relevant physiology, or antigens including outer-surface antigens then important products and therapeutics (e.g., new antigens for vaccines, new targets for antibiotics, and novel bacterial characteristics for diagnostic applications) could be rapidly identified.
Several environmental factors have been identified which influence expression of virulence determinants in bacteria (Mekalanos, J. J., J. Bacteriol. 174:1-7, 1992). For instance, there is a long history of research on the relationship between iron and virulence of bacteria, in particular Shigella (Payne, Mol. MicroBiol., 3:1301-1306, 1989), Neisseria (Payne and Finkelstein, J. Clin. Microbiol., 6:293-297, 1977) and Pasteurella (Gilmour, et al., Vaccine, 9:137-140, 1991).
Other environmental signals that have been shown to control the expression of coordinately regulated virulence determinants of a wide variety of bacteria in plants and animals include phenolic compounds, monosaccharide, amino acids, temperature, osmolarity, and other ions (Mekalanos, J. Bacteriol., 174:1-7, 1992).
Bacterial pathogens that enter an animal host through the intestine (i.e., oral route) encounter numerous host environment components and conditions that may affect bacterial physiology and expression of virulence factors. These components and conditions include bile, bile acids or salts, stomach pH, microaerophillic conditions (the intestine has high CO.sub.2, and low O.sub.2), osmolarity and many others yet undefined. Invasive enteric pathogens require de novo protein synthesis to accomplish internalization (Headley and Payne, Proc. Natl. Acad. Sci. USA, 87:4179-4183, 1990). Therefore, bacteria may optimally produce these invasive factors only in response to certain environmental signals not ordinarily present in vitro. This hypothesis is supported by the recent report that antisera raised against conventionally grown C. jejuni had only a marginal effect on blocking in vitro internalization (Konkel, et al., J. Infect. Dis., 168:948-954, 1993). However, immunization of rabbits with extracts of Campylobacter grown in the presence of epithelial cell monolayers, a condition enhancing invasiveness, resulted in production of an antiserum that markedly inhibited the internalization of the bacteria.
Researchers have been studying growth of bacteria in the intestinal environment to identify relevant virulence factors. For example, Campylobacter strain 81-176 grown in rabbit ileal loops expresses proteins not expressed under conventional laboratory in vitro culture conditions (Panigrahi, et al., Infect. Immun., 60:4938-4944, 1992). New or enhanced synthesis of proteins has been seen in Campylobacter cultivated with INT 407 cell monolayers as compared to bacteria cultured in the absence of the epithelial cells (Konkel, et al., J. Infect. Dis., 168:948-954, 1993). Furthermore, these changes were temporally associated with increased invasiveness of C. jejuni. Other changes such as cellular morphology, loss of flagella, expression of a new outer membrane protein and alteration in cell-surface carbohydrates were induced or enhanced in an avirulent strain of C. jejuni when passed intravenously and chorio-allantoically through chick embryos (Field, et al., J. Med. Microbiol., 38:293-300, 1993).
Other intestinal components, such as bile acids or salts, are known to be inhibitory for some bacteria, but the bile acids may play another role by affecting virulence expression by the bacterium. Is Pope and Payne (93rd Am. Soc. Microbiol., B-147, 1993) reported that Shigella flexneri cultured in broth containing sodium chenodeoxycholate demonstrated 3 to 5-fold enhanced infectivity of HeLa cell monolayers. They reported, however, that other bile salts and detergents including cholate, glycocholate, taurodeoxycholate, the CHAPS series, digitonin and Triton X100 and sodium salts thereof, had no effect on the invasiveness of S. flexneri. Moreover, their broth containing chenodeoxycholate also had no effect on the invasiveness of E. coli or other avirulent strains of shigella.
Synthesis of new proteins by S. flexneri is also induced by altering pH, temperature and ionic composition of the growth medium (Mekalanos, J. Bacteriol., 174:1-7, 1992).
PCT application publication number WO 93/22423, published Nov. 11, 1993, discloses methods for growing bacteria on lipids, such as phosphatidylserine, or mucus and for the isolation of proteins whose expression is enhanced by growth in the presence of phosphatidylserine. This reference neither discloses nor suggests methods of the present invention for producing enteric bacteria having enhanced virulence or antigenic properties.
Vaccines against many enteric pathogens, such as Campylobacter and Shigella, are not yet available but the epidemiology of these disease agents makes such vaccines an important goal. Shigellosis is endemic throughout the world and in developing countries it accounts for about 10 percent of the 5 million childhood deaths annually due to diarrhea. Campylobacter, although only recently identified as an enteric pathogen is now recognized as one of the major causes of diarrheal disease in both the developed and underdeveloped countries. An estimated 400 to 500 million Campylobacter diarrheas occur yearly, and over 2 million cases occur in the United States.
Shigellosis is a consequence of bacterial invasion of the colonic mucosa. The invasion is associated with the presence of a plasmid found in all invasive isolates (Sansonetti et al., Infect. Immun., 35:852-860, 1982). A fragment of this plasmid contains the invasion plasmid antigen (Ipa) genes, Ipa A, -B, -C, and -D. Ipa B, -C, and -D proteins are essential for the entry process (Baudry et al., J. Gen. Microbiol., 133:3403-3413, 1987).
Ipa proteins are logical vaccine candidates although their protective efficacy has not been clearly established. Ipa B and Ipa C are immunodominant proteins (Hale, et al., Infect. Immun., 50:620-629, 1985). Furthermore, the 62 kDa Ipa B protein (the invasin that initiates cell entry and functions in the lysis of the membrane-bound phagocytic vacuole) (High, et al., EMBO J., 11:1991-1999, 1992) is highly conserved among Shigella species. The prolonged illness observed in malnourished children who have no significant mucosal antibody to Shigella Ipa suggests that the presence of mucosal antibody to Ipa may limit the spread and severity of infection.
Though a number of vaccine candidates for Shigella have been tested in animals and humans, a successful one has not been found. In spite of the potential significance of Ipa proteins in virulence, most vaccine candidates developed against shigellosis are based on the lipopolysaccharide antigen, which carries the serotype-specific determinants. A parenterally administered polysaccharide-protein conjugate vaccine has also been developed, but is yet to show significant protection in animals (Robbins et al., Rev. Inf. Dis., 13:S362-365, 1991). A similarly administered ribosomal vaccine does induce mucosal immunity, but its protective efficacy remains to be demonstrated (Levenson et al., Arch. Allergy Appl. Immunol., 87:25-31, 1988).
The pathogenesis of Campylobacter infections is not as well understood as that of Shigella infections. Cell invasion studies in vitro (Konkel, et al., J. Infect. Dis., 168:948-954, 1993) and histopathologic examinations (Russell, et al., J. Infect. Dis., 168:210-215, 1993) suggest that colonic invasion is also important. This conclusion is consistent with the observation that diarrhea caused by Campylobacter may be severe and associated with blood in the stool. These activities may be associated with the immunodominant 62 kDa flagellin protein. A recent report indicates that the presence of flagella is essential for Campylobacter to cross polarized epithelial cell monolayers (Grant et al., Infect. Immun., 61:1764-1771, 1993).
No specific Campylobacter antigens have been established as protective. However, the low molecular weight (28-31 kDa) proteins, or PEB proteins, and the immunodominant flagellar protein are thought to hold promise in this regard (Pavlovskis et al., Infect. Immun., 59:2259-2264, 1992; Blaser and Gotschilch, J. Bio. Chem., 265:14529-14535, 1990). The importance of the flagellar protein is indicated by its association with colonization of the intestine and with the cross-strain protection against infection within Lior subgroups (Pavlovskis et al., Infect. Immun., 59:2259-2264, 1992). However, a flagella protein based Campylobacter vaccine may have to include the flagella protein antigen from the 8-10 most clinically relevant Lior serogroups.
Therefore, objects of the present invention include 1) in vitro culture conditions for culturing or treating enteric bacteria which optimally induce or enhance invasive activities and/or certain cellular characteristics including cell surface characteristics; 2) correlated altered invasiveness or cellular characteristics including surface characteristics with changes in antigenic profiles; 3) increased virulence of these organisms in small animal models; and 4) antisera against organisms with enhanced invasiveness or altered characteristics including surface characteristics that are more effective in neutralizing live organisms used for in vitro or in vivo challenges than antisera prepared against conventionally grown bacteria. This invention addresses these needs and others.
None of the references discussed above teach or suggest the in vitro methods of the present invention nor the vaccines of the present invention comprising antigenically enhanced enteric bacteria. Citation or identification of any reference in this section or any other section of this application shall not be construed as indicative that such reference is available as prior art to the invention.